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The first study that I want to discuss here is by Harrington and others (1999). Although the study is 13 years old, it is a great study showing how complicated natural systems can be, and how the effects of our actions have to be monitored to make sure we pick up their unintended consequences. The study deals with roan antelope, which have always been rare in Kruger. The roan population declined very suddenly in the late eighties, and has since struggled to recover. This study looked at the population of roan antelope from the early eighties up to the mid-nineties to see what was hindering the roan from recovery and if anything could be done to help the population grow.

It's important to bear in mind that the Kruger National Park is located on the southern margin of what would have been the original distribution of roan antelope. As such, roan were probably never really abundant in this area, but nonetheless occurred in greater numbers than they do now.

Harrington and others, in their study, looked at population size for roan and other herbivores from aerial surveys, and also looked at the estimated predator numbers from ranger reports. They also considered the composition of roan herds (male to female, adult to young etc). They then compared their numbers to factors like rainfall and condition of vegetation in the same time period to see if they could match certain events to decline in roan populations. They suggested five possibilities that could have led to the decline, and tested each one against their data. The five possibilities considered were:

1. Habitat deterioration through aridity2. Anthrax (despite immunisation)3. Stress from vaccination against anthrax4. Increased populations of zebra and wildebeest in the roan habitat. Zebra and wildebeest populations increased following the creation of dams and waterholes, which may have led to increased competition for food5. Increased predator populations - with the creation of waterholes, the lion population increased in roan habitats. The lions most-likely followed the zebras and wildebeest. Note that the actual mechanism of the decline is different in 4 and 5 above, but the ultimate cause (more waterholes) is the same.

So, to figure out which of these factors were important in decimating the roan population, Harrington and others first looked at specifics of how the population declined. Between 1977 and 1985 the population was stable at about 300 individuals. In 1986 it grew to 450, after which it declined to 150 between 1988 and 1991. By 1992 it had dropped to 70 and by 1993 it was down to 44 (this is 10% of the high in 1986!). During the decline, both juveniles and healthy adults died, and the percentage of calf survival remained unchanged.

Other important developments noted by the study are the development of dams and waterholes in the habitats favoured by roan:By 1970, five dams and thirty-two waterholes had been built in the northern part of the park in the roans' range. By 1976 there were 9 dams and 46 windpumps, and by 1994, there were 11 dams and 59 windpumps. By 1995, 1 dam and 12 waterholes were closed down.

With regard to potential competitors, zebras numbered 5000 in the northern roan habitat in 1977. This increased to about 14000 during the period between 1986 and 1991.

As for lions in the same area, ranger sightings of prides increased from 10 a year between 1980 and 1985 to 100 a year between 1986 and 1988. This dropped back down to 20 a year in 1989. Investigation of predator kills also showed that roan accounted for a higher proportion of kills than would be expected by their proportion of the herbivore population.

So what caused the roan decline according to Harrington and his coauthors?

The first hypothesis (aridity) seemed unlikely as a controlling factor, because roan declined more than other herbivore populations. Also, a lack of food, as in a drought, is more likely to kill the very old and very young, while those animals in their prime are more likely to survive. But for the roan, healthy adult individuals were also dying.

The second hypothesis (anthrax) was not the cause - an inspection of carcasses revealed that the animals did not have anthrax.

The third hypothesis (stress from vaccination) was also discounted. Of two populations of roan in northern Kruger (one at Mooiplaas and one at Vlakteplaas), only the Vlakteplaas population was vaccinated. Therefore, if stress was the cause, only the Vlakteplaas population would have declined, and the Mooiplaas population would have been unaffected. This was not the case, thus discounting this hypothesis.

The fourth hypothesis (competition with zebras and wildebeest for food) is intriguing. As with lack of food due to aridity, a lack of food due to competition should affect the very old and young before it affect healthy adults. This was not the case. Additionally, zebra populations increased in roan habitats from the early eighties, but roan only started the decline in the late eighties. This hypothesis doesn't entirely explain the lag. This would therefore have been a minor factor.

The fifth hypothesis (increased predation) can explain why healthy adults were dying. It is also supported by the fact that roan were making up a greater proportion of predator kills than their abundance could explain. Also, it explains why roan declined a few years after zebras increased (because it took time for the lions to follow the zebra populations). As such, Harrington and coauthors considered this a likely causal factor in the decline of the roan. Other things like aridity and increased competition for food may then have exacerbated the problem.

So it seemed the problem had been identified: creation of waterholes allowed zebra and wildebeest to move into areas preferred by roan antelope. The zebra and wildebeest were then followed by lions, which increased predation on roan. The problem was made worse by increased competition for food, and also low rainfall in the late eighties.

After the closure of some waterholes in the mid-nineties, some recovery took place. At Vlakteplaas in 1995, 3 out of 4 of the roan herds bred successfully, with this subpopulation growing from 16 to 29 individuals between 1993 and 1997.

So if the problem has been addressed, why hasn’t the roan population shot up in the last ten years? Well, there’s probably still many other aspects of the problem that we don’t understand, like exactly how roan use the resources in their environment, and what effects the increased grazing has on plant species. Ultimately, we need more information on these things before we can figure out a way to increase the population. Finally, because Kruger is at the edge of the range that roan originally lived in, they were probably never very abundant here, and this also makes their conservation here especially challenging.

I read a paper by Kroger and Rogers (2005) a while ago where they look more closely at competition for food between Roan which are specialist foragers and other ungulates which employ a more general foraging strategy ( similar to hypothesis 4 above).

They argue that roan occurring in the northern plains specifically select forage high in nutrients occurring at the boundary between ephemeral wetlands and the savanna matrix. In winter the surrounding vegetation in the landscape has higher carbon to nitrogen ratios which means it will be more difficult to digest and yield less reward in terms of nutrient content (proteins are made from nitrogen) than the vegetation at this boundary. They carried out the study by comparing grazing pressure in this nutrient rich vegetation inside an enclosure (roan enclosure) and outside of it where mixed grazers accounted for the herbivore biomass.

What they found was very interesting:Roan do select for the vegetation that is nutrient rich at very fine scales in the landscape (boundary between ephemeral wetland and savanna matrix). They almost exclusively foraged from this area (remember they are within an enclosure). Similarly outside of the enclosure the nutrient rich vegetation was heavily grazed to a greater extent than within the roan enclosure by large herds of generalist grazers. This heavy grazing outside of the enclosure cannot be attributed to the specialist foraging of the Roan as they occur in negligible densities outside the enclosure but suggests that there is indeed competition between roan a specialist grazer and other general mixed grazers. This makes this competition an important factor to consider in the decline of the roan population.

Interestingly, the large herds of general grazers in this region can be explained by the prevalence of waterholes at this wetland boundary zone as it is easier to develop pans and waterholes as the water lies closer to the surface. So what they are saying is that the choice of location of these waterholes-near to this boundary zone important, for maintaining populations of this specialist grazer were impacted more severely as the waterholes which were opened close by attracted the large herds of other grazers which then began to out-compete Roan for access to these high nutrient resources (essential to them especially in winter periods).

Good work Rockhound!Coincidently I will also discuss some findings on the rare antelope in Kruger from new research done in Kruger, in an upcoming post of my "Kruger Ecology" topics. So keep an eye out for that as well.

Crested Val: according to the sanparks website, there were 90 in 2010/2011. I'm not certain at what number the population is viable (i.e. with a low risk of going extinct in the park).

BillyF: The zebra and wildebeest seemingly prefer permanent water sources (although in other places in the world zebras live in semi-deserts).

Interestingly, the roan population fared better in wet years than dry years. One suggestion in the study is that roan were more susceptible to being killed by lions when using the waterholes in dry years.

Oddesy: Yes, I got the impression from that paper that roan were very fussy eaters! They prefer to only graze at the boundary between ephemeral (temporary) wetlands and savanna matrix, whereas other generalist grazers may use the same boundary area because of its high nutritional content (sodium and nitrogen), and then move on to savanna when they need to.

That study also mentioned that fire may affect the occurrence of the boundary areas that roan favour, meaning that waterhole positioning and fire use may be two things that can be controlled to maintain areas suitable for roan.

Ifubesi: thanks for letting me know about your topic! I will keep an eye out for it.

The fourth hypothesis (competition with zebras and wildebeest for food) is intriguing. As with lack of food due to aridity, a lack of food due to competition should affect the very old and young before it affect healthy adults. This was not the case. Additionally, zebra populations increased in roan habitats from the early eighties, but roan only started the decline in the late eighties. This hypothesis doesn't entirely explain the lag. This would therefore have been a minor factor.

This is interesting - surely you wouldn't expect the Roan numbers to decrease the day the zebra moved in. Surely it would make sense if only a few years later the zebra started to get to such a number and eventually reach a threshold where it would start negatively impacting the Roan population than rather from day 1 wehn the zebra started moving in. To me the way they have tried to disproove this hypothesis doesn't work for me.

Furthermore no mention has been made to water use outside of the park and the fact that the water table in Kruger Park has decreased so drastically in the last few decades (due to water being dammed, or farmeing or used for forestry) that it has impacted on vegetation composition inside the park. This is an area in which so many studies in the park fail to include yet has potentially the greatest impact of them all.

Up here in Botswana, Roan favour the drier areas as they do in Kruger. You will drive through 100''s of km of Mopani and in the areas where there is no water for 10's of km in any direction, you wont see much except for Roan tracks, elephant tracks, hyena tracks, steenbok tracks and if you are lucky leopard or wild dog tracks.I have been helping out on a project with a Masters Student who is based up in the Linyanti concession. In this concession and others I have been to, in the area, the Roan favour the Cathedral Mopani area and infact I was lucky to see some the day before yesterday in this exact habitat type.At this time of year they seem to be competing with no other herbivores. THey survivie in areas you wouldnt think possible and their only real worry from predators in this thick Mopani would be from hyenas.

I would say that (and as has beeen mentioned) the Roan Kruger population decline is not due to one factor but a combination of being outcompeted by zebra during a time of high aridity and high predation. Predation that Roan populations very rarely see. The only dead Roan I have heard of up here are numbering 4 individuals all preyed apon by lions or leopard while they were missioning out of their preffered habitat to go and drink water at the Linyanti River.

Last edited by o-dog on Mon Jul 09, 2012 3:51 pm, edited 1 time in total.

In particular I enjoyed looking at how the conclusions were drawn from different hypothesis. This is a wonderful post for anyone wanting to follow the disipline of research. Its not just about conclusions drawn, but also about how they are drawn.

Oddesey, great to see you around So happy to see you following your dream.

The bird doesn't sing because it has answers, it sings because it has a song.

O-dog, yes, interesting point. You are definitely right in saying several factors contributed to the decline. The influx of a large grazing population can do things like change plant species populations and grass length - roan needing grass cover to conceal their young. So there could be a lag between the zebra influx and roan decline. Perhaps the strongest point in favour of predation playing a larger role than food competition is that healthy individuals (that we would predict to be able to survive a tough season) were also affected very suddenly.

Excellent topic, with fascinating contributions. It seems that the roan populations are slowly increasing, and hopefully we may see a couple hundred in the next few years. During the early 90's, I saw them a few times, but no luck in Kruger since then. I believe the majority of sightings are around the Nshawu dam and waterholes?